The invention relates to a process for producing solid particles from a liquid medium, in which the liquid medium is formed into a liquid jet and is divided into defined sections in such a way that the sections continue to move in the direction of the liquid jet and enter an environment which brings about curing, and thus form the solid particles.
The invention also relates to a device for producing solid particles from a liquid medium, having a nozzle from which the liquid medium emerges as a cohesive liquid jet, having a dividing device, which is moved continuously in one direction in a cutting plane, for producing defined sections of the liquid jet, and having a collection vessel which is arranged in the direction of the liquid jet and is associated with a hardening device for the sections of the liquid jet.
The production of solid particles from a liquid medium is desirable in numerous applications. One significant application area is the encapsulation of chemically or biologically active materials in particles which are generally spherical and in which the chemical or biological activity of the material is maintained, yet the material, being encapsulated, can be handled so that it can be used and retrieved in a controlled manner, for example from a liquid. Suitable liquid media for forming spherical particles together with a hardening medium include ionically crosslinking gels, thermally crosslinking gels, polymer-containing liquids and similar systems. Examples of ionically crosslinking gels are sodium alginate, which sets immediately in a bath containing calcium ions, carrageenan, which hardens with potassium or potassium ions, etc. An example of a thermally crosslinking gel is agar agar, which hardens when the temperature falls considerably, so that the environment bringing about hardening simply has to exhibit a lower temperature in liquid or gas form. In other systems, crosslinking takes place at higher temperatures, so that drops in, for example, a falling tower simply have to be brought to a higher temperature for curing. Furthermore, the use of monomer liquids which together with a monomer polymerize in a hardening liquid or are excited to homopolymerization by the hardening medium is also known. Furthermore, curing by UV radiation is also known.
The production of the portions of liquid medium in a simple manner by controlled dropping from a nozzle cannot be used for commercial production, owing to the low throughputs. Furthermore, it is not possible for the droplets to be smaller than a relatively large minimum size.
It is known from DE 38 36 894 A1 for a larger number of nozzles to be arranged on a common nozzle carrier which is made to vibrate by a vibrator, so that the drops are detached from the nozzle earlier. In this way, it is possible to produce smaller drops. In the vibratory process, the minimum drop sizexe2x80x94and therefore bead sizexe2x80x94which can be achieved is dependent on the viscosity and the surface tension of the liquid medium. Therefore, any desirable reduction in the size of the substantially spherical particles cannot be achieved for media with a relatively high viscosity. Furthermore, the throughput of the liquid medium through the nozzles is limited unless considerable outlay involving a large number of nozzle apertures is accepted.
A dramatic improvement for the production of solid particles from a liquid medium is given by the abovementioned device which is known from DE 44 24 998 C2. In this device, a solid jet of liquid is formed, which is then subdivided into regular sections by a rapidly moving, preferably rotating dividing device. If the hardening device is situated at a certain distance from the dividing device, the surface tension of the liquid medium leads to the sections forming substantially spherical droplets, so that spherical beads are formed after curing. Since natural drop formation is no longer used, but rather the formation of drops is forced by the dividing device, it is possible to achieve a high throughput of material and, furthermore, to set any desired drop size.
The productivity of a device of this nature could be further optimized if the liquid is allowed to emerge from a plurality of nozzles in a plurality of liquid jets and can be divided by the dividing device which is common to all the nozzles, cured in a common hardening device and collected together.
The use of a rapidly moving dividing device leads to an intermediate section which substantially corresponds to the thickness of the dividing device, for example a rotating wire, being removed from the liquid jet. The material removed thus forms a cutting or spray loss, which should be minimized in order to improve the productivity of the device. Tests have shown that the cutting or spray loss is relatively high with a vertically positioned nozzle and a dividing device which has a horizontal cutting plane. Since the liquid in the liquid jet advances considerably during the cutting operation, i.e. passage of the dividing device through the cross section of the liquid jet, the intermediate section which is removed from the liquid jet does not correspond to a right-angled cylindrical section of the jet, but rather to a cylindrical jet section with inclined end faces, with the result that the volume of the intermediate section removed is increased considerably. It has therefore been proposed for the cutting plane of the dividing device to be inclined in accordance with the flow velocity of the liquid jet in order, in this way, to achieve a resultant rectangular cut through the liquid jet. However, an arrangement of this nature cannot readily be achieved with a large number of vertically positioned nozzles which are arranged, for example, on a common radius, since the angle required in each case can only be set at one location of the dividing-device cutting element, which revolves in an inclined plane.
The invention is based on the problem of improving a device of the type described in the introduction in terms of the liquid throughput which can be achieved and the solid particles which can be produced therewith and with a view to reducing the cutting or spray losses.
Working on the basis of this problem, a process of the type described in the introduction is characterized in that a plurality of liquid jets are formed, which are divided in the same direction of movement, and in that each of the liquid jets forms an acute angle with the direction of movement.
Accordingly, a device of the type described in the introduction, according to the invention, is characterized in that a plurality of nozzles are provided, beneath which a common dividing device is arranged, and in that the nozzles are inclined with respect to the direction of movement in such a way that each of the liquid jets emerging from the nozzles forms an acute angle with the direction of movement of the dividing device.
In contradistinction to the known systems of the generic type, in which the nozzles were always aligned in the vertical, so that a liquid jet falling straight down was formed, the nozzles according to the invention are inclined with respect to the vertical. This makes it possible for the resultant cutting path of the dividing device through the liquid jets to run perpendicular to the longitudinal direction of the liquid jet in the cutting plane, and all the nozzles can be at the same distance from the cutting plane.
Since the optimum angle between liquid jet and direction of movement is in any case dependent on the flow velocity of the liquid jet, it is expedient if the inclination of the nozzles is adjustable.
To remove a minimum intermediate section in order to separate the defined sections of the liquid jet, the nozzles may be oriented perpendicular to the cutting plane in the direction running perpendicular to the direction of movement of the dividing device.
It is particularly preferable if the dividing device can be moved in rotation and the nozzles are arranged on the same radius with respect to the rotation axle. The direction of movement in which the dividing device moves continuously is therefore a circular path and, for each cut through a liquid jet, is defined by the relevant tangent on the circular path. Alternatively, it is also possible for cutting elements of the dividing device to be moved substantially in translation, for example as wires clamped in a revolving conveyor, in which case only the carrying section or the return section of this conveyor is used for the cutting operation.
The device according to the invention may expediently be accommodated in a sealed housing, so that the entire arrangement can also be sterilized and can be sterile-sealed, in order to satisfy any demands made in terms of particularly clean and, if appropriate, sterile manufacture. It is therefore readily possible to comply with the cGMP-FDA requirements.
In the device according to the invention, it may be expedient for some of the nozzles, that is to say at least one nozzle, to be designed as a cleaning nozzle which is fed with a cleaning liquid. The cleaning liquid prevents encrustation on the cutting tools and may be designed in such a way that the curing operation is not interfered with even when a curing liquid is being used.